1. Field of the Invention
This invention relates to implantable cardiac pacers having programmable parameter control and, more particularly, pacers having digital circuitry and improved means for receiving externally generated digital parameter control signals.
2. Description of the Prior Art
In the field of cardiac pacers, as well as other implantable types of biomedical devices, there is an increased appreciation of the role to be played by digital type circuitry. The reasons for this are primarily that the digital technology is a very accomplished technology, and it is well understood that digital technology provides a number of advantages, one of the primary ones being that of efficiency and accuracy of control. In the area of heart pacers, producers have historically preferred simple analog embodiments, for the good and sound reasons that reliability of performance is of utmost concern, and that it is very important to minimize power drain from the battery source. However, with improvements in both digital techniques and means for fabricating very compact, relatively low power drain circuitry, digital embodiments are now being considered more favorably. Another reason for considering digital embodiments in heart pacers is that the batteries, or power sources, are now much improved, and the slightly higher power drain of digital circuitry can be more readily tolerated.
The patent literature has for some years now disclosed digital forms of cardiac pacers as well as means for programming same by the pickup of an externally generated program signal. U.S. Pat. Nos. 3,805,796 and 3,833,005 are representative of the prior art. However, the known prior art has a number of shortcomings in the area of reliably introducing parameter control data to an implanted pacer. For example, in prior art arrangements, the externally generated parameter signal, assuming it is accurately received, must be converted after it has been received so as to be in a proper logic form for control of the pacer circuitry. Likewise, in prior art arrangements, the accessing or "addressing" of the pacer is conducted independently of the receipt of the parameter data, such that even though the pacer is reliably isolated from being mistakenly re-programmed by unwanted signals and/or interference, there is no control over ensuring accurate data input after the access has been gained. In other words, even after access has been gained, interference could be present which would cause improper parameter data input to the pacer, or the pacer circuitry itself could improperly receive and/or decode the parameter data from the external source. The prior art has thus retained the problem of properly inputting data to an implanted pacer, especially the problem of reliably inputting a digital data word which is in form for direct parameter control.